Scientific Core Summary The Scientific Core is designed to successfully recruit and monitor HCV-antibody-negative young injection drug users for HCV infection and to clinically characterize acute infection and identified by testing sera for HCV RNA and seroconversion, and persons with acute infection will be clinically characterized by serial ALT and HCV RNA testing. The infrastructure of the Core is necessary for completion of the first trial of an HCV vaccine in people at risk of HCV infection and will be useful in potential future trials of HCV vaccines. Plasma, serum, and PBMC will be collected and stored in the repository for use in Projects 1-3, as well as for other proposed projects in the Cooperative Research Centers (CRC), including those of Dr.'s Walker, Gale, and Lauer (see LOS provided for their applications). It is also anticipated that these specimens will be shared with NIH- funded investigators outside the CRC. Based on almost 15 years of experience, this Core will be a highly successful component of this CRC.